1. Field of the Invention
This invention relates to multiple-degree-of-freedom fine motion devices and more particularly relates to a magnetically-levitated fine motion device having programmable compliance as well as programmable motion.
2. Description of the Prior Art
It has long been recognized that robot control simply by tracking position goals has many limitations when dealing with real-world environments. Compliance is required; that is, there is a need for ability to yield elastically when a force is applied. There has been a great deal of work in the past aimed at giving robot manipulators some form of compliant behavior, and/or control by tracking force goals, etc. Much of this effort has failed to provide satisfactory performance, and applications to the manufacturing domain have been few, if any. Much has to do with the mechanical nature of the manipulator itself. When compliance or force control of a standard industrial robot is attempted, the results are usually dominated by high masses and inertias, as well as friction effects. These effects are difficult to overcome by the generally weak and poorly performing actuators. Additional problems lie with the effective computational bandwidth of the control system.
An approach to this problem is to divide the robot manipulation task into coarse and fine domains. That is, the manipulator itself has redundant coarse and fine degrees of freedom. Here, some form of endpoint sensing is used to measure the directly relevant task parameters and to guide the manipulator system to achieve the desired goal. This paradigm is described in R. L. Hollis, and M. A. Lavin, "Precise Manipulation with Endpoint Sensing," International Symposium on Robotics Research, Kyoto, Japan Aug. 20-23, 1984, and IBM J. Res. Develop. 29, pp. 363-376, July,1985.
For an extremely wide range of robotic assembly tasks especially in the electronics industry, it is only necessary to provide fine compliant motion over limited distances. e.g. fine compliant motion over distances of the order of the features on the parts to be manipulated. It is explicity not required to have compliant motion capabilities over the entire range of motion of the manipulator. Thus, in such a coarse-fine system, the coarse manipulator (CM) can be operated in a strict position-controlled mode, while the fine manipulator (FM) attached to it can be operated in compliance mode or force-controlled mode. The mass and moments of inertia of the FM can be several orders of magnitude smaller than those of the CM, and the motion of the FM can be made frictionless. Accordingly, the desired robot behavior is at least theoretically achievable, assuming a near-ideal FM. The ideal FM should include:
.cndot. 6 degrees of freedom (DOF) redundant with those of the CM; PA1 .cndot. minimal mass to avoid adversely loading the CM; PA1 .cndot. very high acceleration to make it possible to respond to vibrational disturbances in the environment and maximize job throughput; PA1 .cndot. minimal static friction, since the presence of static friction causes loss of accuracy and difficulties with control; PA1 .cndot. FM positional resolution much smaller than the CM for high precision; .cndot. PA1 FM motion range as large as possible, to avoid extra motion of the CM; PA1 .cndot. adequate damping. PA1 .cndot. Full 6-DOF compliant fine motion; PA1 .cndot. Very high performance for light payloads; PA1 .cndot. Extreme simplicity, with only one moving part; PA1 .cndot. Novel combination of actuation, support, sensing, and control means; PA1 .cndot. Docking mechanism to allow coils to cool during coarse motion; .cndot. PA1 Noncontact position and orientation sensing with approximately 1 .mu.m resolution; PA1 .cndot. Multiple possible control modes, including active compliance control to mimic the behavior of mechanisms.